1. Field of the Invention
The present invention relates to liquid level sensors. In particular, it relates to optical liquid level sensors capable of withstanding immersion in high temperature liquids.
2. Description of the Related Art
Liquid level sensors are widely used in various fields of art, including civil, military, automotive and aerospace applications. In many situations, liquids are not permitted to fall below a predetermined level, such as in automobile radiators, fluid containers, and engines. In other situations, a liquid level must be monitored to not reach too high of a level, such as in a holding tank. Applications for liquid level sensors are also found where a liquid is extremely corrosive, such as in chemical reagent reservoirs.
Many different types of liquid level sensors are known in the art. Often, the level of a liquid in a vessel is detected through the use of a float with a mechanical or magnetic coupling to an external gauge, an ultrasonic or optical transducer which measures time of flight to deduce the liquid level, or a parallel wire capacitance sensor which monitors the change in the dielectric constant between the wires associated with a change in liquid level.
Conventional optical liquid level sensors are also known. These sensors utilize an optical prism as a sensing component for detecting the presence or absence of liquid. Typically, a light source and photodiode are closely linked to the optical prism. Divergent light from the light source hits a top surface of the optical prism. If there is no liquid around the optical prism, most of the light will come back to the photodiode due to total internal reflection. On the other hand, if liquid submerges the optical prism, far less light will come back to the photodiode.
A particular problem associated with conventional optical liquid level sensors is their inability to withstand high temperatures. This is particularly caused by the close proximity of the sensor's opto-electronic components (light source, photodiode, electronic circuits) to a high temperature environment such as high temperature liquid. Most opto-electronic components work well at room temperature, but experience degradation and/or failure as temperatures rise above 85° C. during work applications. Furthermore, while some light sources, photodiodes, and electronic circuits are claimed to operate at temperatures of up to 125° C. they are very costly and not readily available. In addition, performances and lifetimes of these opto-electronic components are unreliable and often fail at temperatures approaching 125° C.
Clearly, a need for further improvement exists in the art of liquid level sensors, particularly in the production of sensors which are able to withstand high temperatures while still being lower in cost to produce. The present invention provides a solution to this problem. The invention provides an optical liquid level sensor capable of withstanding temperatures higher than conventionally known liquid level sensors. The inventive sensors can withstand liquid temperatures of about 400° C. or greater, and preferably about 1100° C. or greater.
The inventive liquid level sensors provide an inexpensive alternative to conventional liquid level sensors, since less expensive materials may be used. The inventive design avoids the temperature degradation of the electronic components, thus extending the lifetime of the overall sensor apparatus.